© 2000 Heron Publishing—Victoria, Canada
Responses of trembling aspen and hazelnut to vapor pressure deficit in a boreal deciduous forest
E. H. Hogg (1), B. Saugier (2), J.-Y. Pontailler (2), T. A. Black (3), W. Chen (3), P. A. Hurdle (1) and A. Wu (3)
1. Canadian Forest Service, 5320-122 Street, Edmonton, Alberta T6H 3S5, Canada / 2. Ecophysiologie Végétale, Université Paris-sud, 91405 Orsay Cedex, France / 3. Department of Soil Science, University of British Columbia, Vancouver, BC V6T 1Z4, Canada / Received June 3, 1999
Summary
The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen (Populus tremuloides Michx.) and hazelnut (Corylus cornuta Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ecosystem–Atmosphere Study (BOREAS). During the same
period, daytime measurements of stomatal conductance (gs) and leaf water potential (Ψleaf) were made on these species, and sap flow was monitored in aspen stems by the heat pulse method. Weather conditions during
the study period were similar to the long-term average. Despite moist soils, both species showed an inverse relationship between
daytime gs and vapor pressure deficit (D) when D was > 0.5 kPa. Daytime Ψleaf was below –2 MPa in aspen and near –1.5 MPa in hazelnut, except on rainy days. These results are consistent with the hypothesis
that stomatal responses are constrained by hydraulic resistance from root to leaf, and by the need to maintain Ψleaf above a minimum threshold value. Reductions in gs on sunny afternoons with elevated ambient D (maximum 2.3 kPa) were associated with a significant decrease in photosynthetic rates. However, day-to-day variation in mean
carbon assimilation rate was small in both species, and appeared to be governed more by solar radiation than D. These results may be generally applicable to healthy aspen stands under normal midsummer conditions in the southern boreal
forest. However, strong reductions in carbon uptake may be expected at the more extreme values of D (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.
Keywords:
branch bag method, Corylus cornuta, photosynthesis, Populus tremuloides, stomatal conductance, transpiration.
